1. Field of Invention
The present invention relates to a heavy-wall H-shaped steel excellent in toughness and yield strength (abbreviated as "YS", yield point or proof stress) which is suitable for use in structural members such as pillars, beams and the like for a high-rise building. The present invention further relates to a process of making the steel.
In the present invention, the term "wt %" regarding the chemical composition means weight percentage. Herein, the "L-direction" means the rolling direction; the "C-direction" is a direction perpendicular to the rolling direction and the thickness direction; and the "Z-direction" is the thickness direction.
2. Description of Related Art
Hot-rolled H-shaped steels are popularly used for pillars and beams for buildings. For an H-shaped steel, SM490 steel, SM520 steel or SM570 steel (specified in JIS G 3106 as a rolled steel product for welded structure) are widely used. H-shaped steels are directed toward a larger thickness and a higher strength, along with the tendency of building toward greater heights and larger scales. For example, an H-shaped steel is required to have a YS of at least 325 MPa, or more preferably, at least 355 MPa, a yield ratio (YR) of up to 80%, and a high toughness. These properties are expressed by the following formula: EQU Yield ratio(YR)=Yield strength(YS)/tensile strength(TS)
However, an increase in thickness of a steel product generally tends to also lead to a decrease in its strength. In an H-shaped steel having a flange thickness of at least 40 mm, it is difficult to achieve a high strength as represented by a YS of at least 325 MPa or 355 MPa. In order to ensure a high strength by manufacturing the product based on an ordinary hot rolling process, it is inevitable to increase the carbon equivalent (Ceq) of the steel product, thus resulting in a higher welding crack sensitivity (degradation) and a decrease in toughness at the welding heat affected zone (hereinafter referred to as "welding HAZ").
In rolling a heavy-wall H-shaped steel, which must be carried out under an equipment limitation of a small mill load relative to the sectional area of the bloom, it is the usual practice to adopt a small reduction rolling (reduction/pass: 1 to 10%) at a high temperature (at least 950.degree. C.) of a small deformation resistance. Under these rolling conditions, however, grain refinement is insufficient, leading to the problem of difficulty in obtaining a satisfactory toughness.
Manufacture based on the TMCP (Thermomechanical Control Process) is known to ensure satisfactory strength, toughness and weldability in heavy-wall H-shaped steel. For example, Japanese Examined Patent Publication No. 56-35734 discloses a manufacturing method of a flange-reinforced H-shaped steel, that includes the steps of hot-rolling a bloom into an H-shaped steel, rapidly cooling the resultant H-shaped steel from the flange outer surface to a temperature range of from the Ar.sub.1 transformation point to the Ms transformation point, and then air-cooling the steel, thereby forming a fine, low-temperature-transformed microstructure. Japanese Examined Patent Publication No. 58-10422 discloses a manufacturing method of a high-strength steel excellent in workability that includes the steps of, after heating, applying a rolling reduction of at least 30% at a temperature at least within the range of from 980.degree. C. to the Ar.sub.3 transformation point to cause precipitation of ferrite, and rapidly cooling such that the resultant steel has a ferrite-martensite dual-phase composite microstructure.
In these conventional techniques, however, rapid cooling from the flange outer surface after hot rolling results in considerable differences in strength and toughness on the flange thickness cross-section and in serious levels of residual stress and strain, thus posing many problems upon application to a heavy-wall H-shaped steel.
Japanese Unexamined Patent Publication No. 9-125140 discloses that a certain S content (0.004 to 0.015 wt %) and addition of V and N enables a ferrite refinement effect of VN precipitating during rolling and subsequent cooling, thus giving a heavy-wall H-shaped steel having excellent properties. This publication also discloses that an appropriate combination of rolling conditions in the recrystallization region brings about a further improvement of the refinement effect. In this technique, however, it is necessary to use an S content of at least 0.004 wt % in addition to V and N to achieve the ferrite refinement effect, and as a result, improvement of toughness is limited at least to some extent by production of MnS. A particularly serious problem in such steels is a still insufficient Charpy absorbed energy in the Z-direction.
Japanese Unexamined Patent Publication No. 5-132716 discloses a toughness improvement technique by grain refinement. The grain refinement is achieved by creating inner-grain ferrite by dispersing composite inclusions composed of Al, Ti, Mn or Si composite oxides, MnS and VN. In this technique, however, it is sometimes difficult to disperse oxide particles finely and uniformly. Consequently, the grain refinement is sometimes insufficient. Accordingly, it is difficult to improve toughness in the Z-direction.
When a bending strain is applied to a beam of a building structure by an earthquake or a like high-energy event, stress concentrates in the Z-direction at a junction of a pillar and the beam. With a small Charpy absorbed energy in the Z-direction, such stress concentration causes brittle fracture even from a small deformation. For the purpose of improving seismic resistance, therefore, the Charpy absorbed energy in the Z-direction should preferably be as high as possible.